Cross-coupled Combline Bandpass Filter using Active Capacitance Circuit

In this paper, a cross-coupled combline bandpass filter with active capacitance circuit using BJT is presented. The proposed active combline bandpass filter is composed of 2 normal microstrip resonators and 2 microstrip resonators with active capacitance circuits. The negative resistance of the active capacitance circuit compensates the loss of the microstrip resonator. According to the Q distribution method, we arranged the 2 active resonators with high Q in the middle stages of the filter. The proposed active bandpass filter has very good insertion loss and selectivity with 2 transmission zeros. Moreover, it has good noise figure with common-emitter configuration. Therefore, it can be applicable to the modern wireless communication systems. The measured performance of the proposed filter has center frequency of 1.77 GHz, 5.1 % fractional band width, and maximum 2 dB insertion loss. The measurement shows good agreement with the simulated results.